Evaluating a multi-layered structure for voids

ABSTRACT

A method and apparatus measure properties of two layers of a damascene structure (e.g. a silicon wafer during fabrication), and use the two measurements to identify a location as having voids. The two measurements may be used in any manner, e.g. compared to one another, and voids are deemed to be present when the two measurements diverge from each other. In response to the detection of voids, a process parameter used in fabrication of the damascene structure may be changed, to reduce or eliminate voids in to-be-formed structures.

This application is a continuation application of U.S. patentapplication Ser. No. 11/114,300 filed Apr. 25, 2005 which is adivisional application of U.S. patent application Ser. No. 10/090,262filed on Mar. 1, 2002 now issued as U.S. Pat. No. 6,885,444.applications Ser. No. 11/114,300 and 10/090,262 are hereby incorporatedby reference herein in their entirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to and incorporates by reference herein intheir entirety the following commonly owned U.S. patent applications:

application Ser. No. 09/095,805 entitled “AN APPARATUS AND METHOD FORMEASURING A PROPERTY OF A LAYER IN A MULTILAYERED STRUCTURE”, filed Jun.10, 1998 by Peter G. Borden et al. which is now issued as U.S. Pat. No.6,054,868;

application Ser. No. 09/521,232 entitled “EVALUATING A PROPERTY OF AMULTILAYERED STRUCTURE”, filed on Mar. 8, 2000 by Peter G. Borden et al.which is now issued as U.S. Pat. No. 6,812,047;

application Ser. No. 09/544,280 entitled “AN APPARATUS AND METHOD FOREVALUATING A WAFER OF SEMICONDUCTOR MATERIAL”, filed Apr. 6, 2000 byPeter G. Borden et al., which is a continuation of Ser. No. 09/095,804filed Jun. 10, 1998 and now issued as U.S. Pat. No. 6,049,220;

application Ser. No. 10/090,287 entitled “IDENTIFYING DEFECTS IN ACONDUCTIVE STRUCTURE OF A WAFER, BASED ON HEAT TRANSFER THERETHROUGH”filed on Mar. 1, 2002 by Peter G. Borden and Ji-Ping Li, now issued asU.S. Pat. No. 6,971,791; and

application Ser. No. 10/090,316 entitled “AN APPARATUS AND METHOD FORMEASURING A PROPERTY OF A LAYER IN A MULTILAYERED STRUCTURE” filed onMar. 1, 2002 by Peter G. Borden and Ji-Ping Li, now issued as U.S. Pat.No. 6,958,814.

BACKGROUND

Damascene structures in semiconductor substrates are so-named becausethey consist of metal lines formed in narrow grooves, as illustrated bythe structure shown at the bottom of FIG. 1. These metal lines may be ofwidth W <0.15 μm and height H >0.5 μm, with an aspect ratio that mayexceed 3:1 (ratio of height to width). The grooves may be formed withina dielectric layer that has a total thickness of 1 μm and a thicknessbetween the bottom of the grooves and the bottom of the dielectric layerof T =0.5 μm.

Such damascene structures are typically formed in a multi-step process,of the type shown in FIG. 1. First, in step 110, photoresist layer 101is formed on insulator layer 102 over substrate 103. Insulator 102 is amaterial such as silicon dioxide, and substrate 103 is silicon. In step111, photoresist layer 101 is patterned, forming grooves 104 a-f. Thestructure is then etched in step 112, forming grooves 105 a-f in theinsulator layer 102. Note that the grooves are less deep than thethickness of the insulator 102. The photoresist layer 101 issubsequently stripped. In step 113 the structure is coated with abarrier layer of a metal such as tantalum, followed by a seed layer of ametal such as copper, indicated as combined layers 105 bs. The copperseed layer provides a conductive coating to allow electroplating of athick copper layer onto the structure in step 114, that material beingshown as layer 106. The seed layer may be 1000 Å thick on the surface,but only 100-200 Å thick on the walls of the grooves. Similarly, thetantalum layer may be 250 Å thick on the surface, but only 50 Å or lessthick on the walls of the grooves. The tantalum layer prevents thecopper from diffusing into the underlying layers; hence its name“barrier”, and also improves adhesion of the copper to insulator 102. Instep 115 the electroplated layer 106 is polished away, leaving a fill ofcopper in the grooves.

The yield of this process depends on the thickness t of each sidewall ofeach groove. This is a parameter called sidewall coverage. If thesidewall coverage is too thin, then the coating may be discontinuous, oreven non-existent. It then acts as a poor nucleating surface for thesubsequent electrodeposition of subsequent thick layer 106, causingproblems such as void formation. Specifically, voids may arise if theplating process does not begin properly on one of the sidewalls. Forexample, filling may not start at the bottom of a groove, resulting in a“bottom” void. As another example, if a groove fills from the side, butnot the bottom, then a “center” void may occur.

Such voids Va-Vc (FIG. 1) act as breaks in the metal lines 107 a-107 c,either preventing current flow, or constricting current flow to thepoint where the line locally overheats and fails. If the coating is toothick, the top of the groove may close off, preventing adequatecirculation of electrodeposition electrolyte, resulting in poor fillingof the grooves which can also result in voids. This problem is furtheraggravated as the technology advances, and the grooves become deeper andnarrower.

Voids in a semiconductor wafer can be generated in other ways as well.For example, voids in the metal line can be opened during chemicalmechanical polishing (CMP). Also, voids may be created between a metalline and the underlying dielectric layer due to volume contraction thatoccurs after annealing. Furthermore, stress- and electromigration voidstypically nucleate at intersections of metallization grain boundariesand the passivation/interconnect interface, and can form as aconsequence of current flowing through the line, resulting in failurethat may occur months after manufacture.

A prior art method for detection of voids is to electrically probe longruns of metal lines, often after electrical stress. However, this is notuseful in production because it only finds the voids long after theprocess has been completed. In addition, it requires contact to theproduct wafer, and such long runs for void testing are not readilyavailable on product, because they require considerable wafer area.

Voids may also be seen using FIB-SEM (Focused Ion Beam Scanning ElectronMicroscopy). A focused ion beam mills out a section of the line, and ascanning electron microscope is used to view the section. This method isslow and destructive, and views such a small area that it is only of usein cases of extremely high void density, or if the void location hasbeen found by other means such as electrical testing.

An article at the websitehttp://www.micromagazine.com/archive/00/01/prods.html published inJanuary 2000 describes a “Wafer Inspection System” available fromSchlumberger of San Jose, Calif., as follows “The Odyssey 3000 uses anE-beam-based voltage-contrast technology to identify yield-killingelectrical defects that are difficult to detect using optical systems.Designed for use in sub-0.15-μm processes, the system detectselectrical, submicron particle, and pattern defects. Metal stringers,poly gate shorts, and copper damascene voids are also detectable.Real-time off-line defect review with simultaneous inspection eliminatesthe need to buy off-line inspection tools. The system accommodates300-mm wafers and features an advanced graphical user interface tosimplify recipe setup. Die-to-die comparisons also enable users to carryout rapid defect review.”

However, such a system has limited application because it can only lookat exposed layers. In many cases, lines with voids may be buried under adielectric layer, and the formation of the dielectric layer may in facthave been the cause of the voids. The electron beam charges thedielectric, preventing its use in such cases. In addition, thevoltage-contrast method is only of use in cases of extreme voiding,where the lines are almost completely broken. This is seldom the case;often the voids are smaller, but these small voids are critical todetect because they may grow into sources of failure due to factorsdescribed earlier, such as electromigration and stress.

SUMMARY

A method and apparatus in accordance with the invention measureproperties of two layers of a damascene structure (e.g. a silicon waferduring fabrication), while scanning, and use the two scanningmeasurements to identify a location as having voids. The two scanningmeasurements can be used in any manner, although in one embodiment, thetwo measurements are compared to one another (either automatically bycomputer during the scanning or visually by an operator after thescanning). Voids are deemed to be present when the two measurementsdiverge from each other. The two measurements can be either of twolayers that are adjacent to and in contact with one another or of layersthat are separated from one another by one or more intermediate layers,depending on the embodiment. In response to the detection of voids, aprocess parameter used in fabrication of the damascene structure may bechanged, to reduce or eliminate voids in to-be-formed structures,thereby to perform process control in real time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates structures formed in a prior art process of waferfabrication.

FIG. 2 illustrates, in a flow chart, a method for detecting voids inaccordance with the invention.

FIG. 3 illustrates, in a graph 130 shown relative to a structure 115,the measurements S1 and S2 in the presence and absence of voids.

FIG. 4 illustrates, in a perspective view, a portion of a structurehaving a number of traces in a region illuminated by a beam ofelectromagnetic radiation in accordance with the invention.

FIG. 5 illustrates, in an elevation view in the direction 1B of FIG. 4,the relationship between the diameter Dp of the beam and the pitch pbetween the traces.

FIG. 6 illustrates the direction of polarization of the beam relative tothe traces when respectively measuring a property of a dielectric layerunderneath the traces.

FIG. 7 illustrates the direction of a heating beam (used in oneembodiment in addition to the beam of FIGS. 6 and 8) relative to thetraces when measuring a property of the traces.

FIG. 8 illustrates a beam having a parallel polarized component and aperpendicular polarized component for use in measuring properties of thetraces and of the dielectric layer contemporaneously.

FIG. 9 illustrates, in a block diagram, use of the measurement apparatusof FIG. 12 in a processing unit for fabrication of semiconductor wafers.

FIG. 10 illustrates, in a flow chart, one embodiment that implements themethod of FIG. 2.

FIG. 11 illustrates a two dimensional map of a wafer showing dies thathave different voiding.

FIGS. 12A and 12B illustrate, in graphs, plots of measurements performedin the embodiment illustrated in FIG. 10.

FIG. 13A illustrates, in a top down view through a microscope, a numberof trench arrays in a reference wafer and the relative size of a laserbeam used in a method of the type described herein.

FIG. 13B illustrates, in a graph, correlation of a measured signal tothe presence or absence of voids.

FIGS. 13B-1-13B-3 illustrate in cross-sectional views structures thatgenerates signals 1711-1713 respectively.

FIGS. 13C and 13D illustrate in a two dimensional graph and a threedimensional graph respectively, an area scan obtained by a method of thetype described herein.

FIG. 14 illustrates, in a block diagram, one embodiment of a measurementapparatus of this invention.

FIG. 15 illustrates, in a high-level block diagram, a circuit includedin measurement apparatus of FIG. 14 in one embodiment.

FIGS. 16A-16D illustrate, in drawings (FIGS. 16A and 16C) and graphs(FIGS. 16B and 16D), methods for measuring voiding in a single(isolated) metal line.

DETAILED DESCRIPTION

One embodiment measures (e.g. see acts 122 and 123 in FIG. 2) propertiesof two layers of a structure, and uses the two measured signals todetermine existence of voids (see act 124). Measurements (as illustratedby signals S1 and S2 in FIG. 3) change depending on the presence andabsence of voids (see the change in signals S1 and S2 in regions 131 and132 respectively) when a scan is performed across a wafer. In theexample illustrated in FIG. 3, signal S1 reduces by the amount dS1 andsignal S2 increases by an amount dS2, so that a separation D1 between S1and S2 in the presence of voids is reduced to the separation D2.

The above-described acts 122-124 for finding voids are effective even ina damascene structure that is formed in the normal manner, e.g. byetching grooves in a dielectric layer and forming conductive traces(also called “lines”) inside the grooves as illustrated in FIG. 1.During fabrication of the damascene structure, voids can be formedbetween the traces and the dielectric layer, e.g. by failure of aconductive material (that is used to form a trace) to completely fill agroove, and the above-described acts 122-124 detect such voids. Inresponse to the detection of voids, a process parameter used infabrication may be changed to reduce or eliminate voids in to-be-formedstructures thereby to effectuate process control in real time.

Therefore, the above-described acts 122-124 detect voids in anon-destructive manner, and with high resolution when considering thatsuch voids are often buried underneath the conductive lines, in whichcase they are difficult to see by conventional methods.

The above-described measurements of two signals may be used (see act 124in FIG. 2), in any manner well known in the art, to determine existenceof voids. For example, the measurements may be displayed on a screen ofa computer, and a human operator may notice a change in measurements,thereby to detect voids. Depending on the embodiment and the specificexample, the change from D1 to D2 (FIG. 3) can be as great as an orderof magnitude, so that detecting such a change is easy. Moreover, aseparation D between the two electrical signals generated in acts 122and 123 can be plotted to obtain a two dimensional image of structure 10(see FIG. 4) as a whole, so that the image indicates the presence andabsence of voids across structure 10. Instead of a two-dimensionalimage, the two electrical signals from acts 122 and 123 can bethemselves plotted in a graph along the y axis, with the x axisrepresenting regions in structure 10. Note that presence/absence ofvoids can also be detected automatically (by a computer that monitorsseparation D between measurements at each location).

Specifically, divergence of the two signals can be determined bycomparing separation D at various locations of a wafer under evaluation(also called “production wafer”), either in an absolute manner or in arelative manner. In one embodiment, absolute comparison is performed asfollows: voids are deemed to exist at a location if separation D of thetwo signals is greater than a predetermined amount Dmax, which isdetermined from test wafers (which are wafers known to have voids andwafers known to have no voids). Therefore, although in FIG. 3, regions131 and 132 are illustrated as being portions of a single wafer (e.g.obtained after step 115 of the prior art as described above), acomparison can be made across wafers to determine if the two signalsdiverge from each other (to implement act 124 of FIG. 2).

In an alternative embodiment, a relative comparison is made betweendivergence D1 at one location as compared to divergence D2 at anotherlocation (both locations being in the same structure) as illustrated inFIG. 3. In one example of the just-described alternative embodiment,voids are deemed to be present if separation D1 at the current locationis at least a predetermined percentage (e.g. 50% or 1000%) of theseparation D2 in another location (e.g. a location that does not haveone of the two layers in which case the predetermined percentage may be50% for example, or a location that does not have a void in which casethe predetermined percentage may be 1000% for example).

Acts 122 and 123 can be performed contemporaneously (e.g., just before,during or just after each other). Moreover, acts 122-124 can beperformed in-situ during wafer fabrication (see method 120), immediatelyafter creation of a region (see act 121; e.g. after chemical mechanicalpolishing), so as to adjust the fabrication process (see act 125) anddiscard the wafer if voids are determined to be present. The processbeing adjusted in act 125 can be, e.g. a sidewall deposition process ora fill (plating) process, or both. The specific adjustment to be madedepends on the process, and knowledge of which parts of the process aremost susceptible to change, and which parts have the strongest effect.If no voids are found, fabrication is continued with the wafer.Depending on the embodiment, the just-described measurements may beperformed repeatedly (either before or after act 124), so that a numberof regions (which may be, for example, an integral multiple of thenumber of dice) of a wafer are evaluated before the wafer is processedfurther (which further processing may be performed in the normalmanner).

Act 124 in FIG. 2 may be implemented by performing actions other thancomparison. For example, in other embodiments, measurements from the twoacts 122 and 123 are used in a formula (which can be, for example,simple multiplication) and the result is used to detect voids (e.g. bycomparison against a threshold which may be determined from “reference”structures). Moreover, in one embodiment, the amount of adjustment madein Act 125 depends on the magnitude of divergence D. Specifically, theamplitude of separation of the two signals may be used as a measure ofthe size of the voids.

Depending on the embodiment and on the materials used in a wafer, theabove-described change dS2 in signal S2 can be negligible (i.e. notnoticeable), as compared to the change dS1 in signal S1. For thisreason, one embodiment monitors a change in signal S1 alone instead oflooking for changes in both signals S1 and S2. One variant of thisembodiment does look at signal S2 but only after finding a change insignal S1, and then only as a confirmation, before determining thatvoids exist at this location. Instead of looking for confirmation by achange in S2, such an embodiment may use some other indication, e.g. acorresponding change in a global property (as opposed to a localproperty) may be used to rule out voids as the cause for the change inS1. Detecting a divergence in two signals (e.g. S1 and S2) at the sameposition reduces (and even eliminates in one example) the possibility offalsely determining that voids are present. Such a false measurement maybe obtained when monitoring only one signal, e.g. by a scratch on thesurface that changes both the dielectric and metal thickness.

Moreover, depending on the type of properties being measured and thematerial in the two layers, one or both of the measured signals may bescaled and aligned to one another. Also depending on the embodiment, anyproperties (also called “dimensional properties”) of the two layers thatdepend on the dimensions of the features in the wafer may be measured(in acts 122 and 123 of FIG. 2) by any method well known in the art.Note, however, that one or more non-dimensional properties (such asconductivity and adhesion of metal to the dielectric) are measured inalternative embodiments.

In one embodiment, a signal indicative of the thickness of a dielectriclayer of a damascene structure is measured and another signal indicativeof the thickness (or cross-sectional area) of the traces supported bythe dielectric layer is also measured. In another embodiment, aresistance measurement measures the line cross-section, and a dielectricmeasurement measures the thickness of the structure. Comparison of thesetwo measurements determines whether a change in resistance is due tothickness variation or line width variation—in the former case the causemay be polishing, in the latter it is a lithography problem.

Instead of the just-described dimensional properties, other propertiesof the materials in the wafer can be measured (in acts 122 and 123). Forexample, in one embodiment, the average resistance per unit length ofthe traces is measured (in any manner well known in the art). Note thatin the just-described example, the measured resistance can be convertedinto cross-sectional area if the composition of the conductive materialused to form the traces is known. In another example, the thermalconductivity of the insulator between the lines is measured. Here againthe resistance signal is a function of thermal conductivity between thelines. The resistance per unit length can be measured using a thermalmethod such as described in U.S. patent application Ser. No. 09/095,805,but as the thickness of the lines tracks the dielectric thickness, sovariations in thermal conductivity (or for that matter, adhesion of thelines to the dielectric) could be measured in a similar manner.

Acts 122 and 123 can be performed in any manner well known in the artfor measuring such properties. One method of measuring resistance perunit length is physical probing of lines. Usually this is done with aserpentine structure with bond pads at each end. The length of the lineis known, and the probed resistance divided by the length gives theresistance per unit length. One way of measuring cross-section is tophysically cut and image the line. A common method is the FIB-SEMdescribed earlier, where a focused ion beam (FIB) is used to mill away acut into the line and then a scanning electron microscope (SEM) is usedto image the cross-section. The thickness of a dielectric layer iseasily measured using a number of methods, such as measuring reflectionvs. wavelength (reflectrometry) or ellipsometry. Moreover, thepolarization methods described in U.S. patent application Ser. No.09/521,232 may be used with patterned lines.

In a first embodiment, act 122 is implemented as follows: a single beamis used as described in U.S. patent application Ser. No. 09/521,232(which is incorporated by reference above) to measure the thickness(e.g. T+H in FIG. 3) of dielectric layer 133, by measuringelectromagnetic radiation reflected by the structure and polarizedperpendicular to the traces. In this embodiment, act 123 may beimplemented, to measure the average thickness (or cross-sectional area)of traces in contact with the dielectric layer, in any manner well knownin the art (including the above-described prior art methods).

In a second embodiment, act 123 is implemented as follows: two beams areused as described in U.S. Pat. No. 6,054,868 (which is incorporated byreference above), wherein electromagnetic radiation in at least one beam(and preferably both beams) is polarized parallel to the traces.Depending on the implementation, the two beams may or may not becoincident. In the just-described second embodiment, act 122 may beimplemented, to measure the thickness of the dielectric layer, in anymanner well known in the art (including the above-described prior artmethods).

A third embodiment uses act 122 as implemented by the first embodimentand act 123 as implemented by the second embodiment. Depending on theimplementation of the third embodiment, one beam may be used commonly inacts 122 and 123, and the additional beam of act 123 may be used toobtain a second measure of the thickness of the dielectric layer(wherein a first measure is obtained in act 122 as described above).Such a second measure may be used to eliminate ambiguity that arisesfrom the sinusoidal nature of the dielectric thickness signal, asdescribed below.

As described above, such measurements from acts 122 and 123 are used ineach of the first, second and third embodiments, to evaluate the siliconwafer for voids. Note that although the description refers to a wafer ofsilicon, the description is equally applicable to any multi-layeredstructure, such as any substrate that supports a conductive layer, andexamples of a substrate other than silicon wafer include a glass plateand a resin core.

In one implementation, in each of acts 122 and 123 a beam 12 (FIG. 4)illuminates a multi-layered structure 10, and has a diameter Dp (atsurface 13S of structure 10) that is selected to be several times largerthan the width W of a trace 11I. For example, diameter Dp can be 2microns and width W can be 0.15 microns (so that seven lines aresimultaneously covered by beam 12). In this implementation, beam 12merely illuminates region 11 and may or may not be focused on surface13S (which is a surface of structure 10 exposed to a transmissive medium15 such as air). In different embodiments, beam 12 is focused on (a)surface 13S, (b) surface 14S, (c) between surfaces 13S and 14S, (d)surface 16, or (3) above surface 13S. If a heating beam 19 is used asdescribed elsewhere herein to evaluate a property of traces 11A-11N,both beams 12 and 19 are focused on the same surface (which can be anyone of the just-described surfaces) in one embodiment. In anotherembodiment, beams 12 and 19 are focused on different surfaces.

In this embodiment, beam 12 is selected to have a wavelength greaterthan or equal to pitch p between two adjacent traces 11I and 11J. Beam12 does not resolve individual features in region 11 (unlike a scanningmicroscope of the prior art which can resolve the individual features).Instead, beam 12 is used to obtain an average measure of one or moreproperties in illuminated region 11, e.g., of traces 11A-11N, or oflayer 13 or a combination thereof. A single void in one of the tracesunder illumination causes the heat in that line to rise more than theothers and contribute measurably to the signal. Therefore, themeasurement can find isolated lines within the illuminated spot. Thedetermination made in act 124 (FIG. 2) is an indication of the presenceof voids in region 11 as a whole, rather than an indication that a voidexists in a specific trace in structure 10.

Note that even a single line 11A may be under the beam 12, even if beam12 has a diameter much greater than the width of line 11A, or even ifthe diameter of line 11A is comparable to the diameter of beam 12. Insuch a case, a single line 11A provides a signal for light in beam 12polarized along the direction of line 11A in sufficient intensity to bemeasured, and a measurement for voids may be performed in an analogousmanner. In addition, a beam 12 polarized orthogonal to line 11A will notbe reflected by line 11A, and a dielectric thickness measurement may bemade. Note that a measurement along the length of the line may also bemade as discussed below in reference to FIGS. 16A-16D.

As noted elsewhere herein, such a structure 10 contains a number oftraces 11A-11N (A≦I≦J≦N; N being the total number of traces) passingthrough a region 11 (also called illuminated region) of a layer 13.Traces 11A-11N (FIG. 4) are each substantially parallel to and adjacentto the other (e.g., centerlines CI and CJ of traces 11I and 11J that areadjacent to each other form an angle of less than 25° relative to oneanother). Traces 11A-11N have an index of refraction different from theindex of refraction of layer 13, and therefore reflect theelectromagnetic radiation in beam 12 that is directed at region 11. Notethat traces 11A-11N need not be conductive, although in one embodimentthe traces are conductive. Structure 10 can be (but is not required tobe) a wafer of the type commonly used to manufacture integrated circuitdies.

A portion of beam 12 is reflected by region 11, and is used to generatean electrical signal (e.g., by use of a photosensitive element) thatindicates an attribute (e.g., intensity or optical phase) of thereflected portion. The measured attribute in turn is used as an averagemeasure of a property of a layer in region 11. For example, if thejust-described acts 122 and 123 are performed in one region 11, a stage24 (FIG. 12; described below) that supports structure 10 moves structure10 so that a different region is illuminated, and then these acts 122and 123 are repeated.

Two regions in which such measurements are made can be separated fromeach other, e.g., by distance which is same as the diameter Dp of beam12. Alternatively, the two regions can touch each other or even overlapeach other. When overlapping one another, the centers of the two regionsmay be separated by a small fraction of the diameter, e.g., by ( 1/10)Dp or less. Moreover, in this implementation, act 124 may be performedin one of two ways: (1) after acts 122 and 123 are performed, and beforethey are repeated; or (2) at the end, after acts 122 and 123 areperformed repeatedly, for all regions of interest.

In an alternative embodiment, one beam polarized parallel to the linesis used for measuring a property of the lines (e.g. reflectance orconductivity) and one beam polarized perpendicular to the lines is usedfor measuring a property of the dielectric (e.g. adhesion to metal). Inthis alternative embodiment, the two beam measurements provide twoinputs, giving more generalization to the measurement (as compared touse of unpolarized beams, for example). Also, one beam (such as aheating beam) could be a beam other than light, such as an electron beamand the other beam could be an optical beam, depending on theembodiment.

One embodiment involves stepwise movement (“hopping”) from one region toanother region of structure 10 when performing measurements of the typedescribed herein. In the hopping process, stage 24 (FIG. 14) holdsstructure 10 stationary for a moment (e.g., 1 second) while ameasurement is taken in one region, and then moves to another region(e.g., of the same structure). Another embodiment involves continuousmovement (e.g. in a “sweep”) of structure 10 relative to beam 12, whilemeasurements of acts 122 and 123 are made simultaneously, and plotted ina graph as a function of distance (along the x axis) as illustrated inFIGS. 12 and 13 (described below). In some embodiments, measurements usehopping, but the hopping steps are on the order of the size of a spotformed by beam 12 on structure 10, so that the result when plotted as afunction of distance is equivalent to a continuous movement sweep.Therefore, hopping in small steps on the order of or smaller than thespot size results in a scan equivalent to a sweep.

Beam 12 can be linearly polarized, circularly polarized, ellipticallypolarized, nonpolarized or some combination thereof, depending on theimplementation. So, in one implementation, beam 12 is nonpolarized, andone embodiment of act 122 (FIG. 2) generates a single electrical signalfrom the reflected portion. Such an electrical signal (as a whole)provides an average measure of the thickness (T+H) of a layer 13 (FIG.5) that supports traces 11A-11N.

To implement act 122 (FIG. 2), another beam (also called “heatingbeam”), in addition to beam 12 (also called “probe beam”), can be usedto heat traces 11A-11N (e.g., as described in the related patent, U.S.Pat. No. 6,054,868 that is incorporated by reference above). In oneembodiment the intensity of the heating beam (and therefore thetemperature of traces 11A-11N) is modulated at a frequency that issufficiently low to avoid creation of a thermal wave. If so modulated, areflected portion of probe beam 12 is also modulated at thejust-described frequency, in phase with the heating beam's modulation.So in this implementation of act 122 (FIG. 2), the reflected portion ofprobe beam 12 is measured by use of a lock-in amplifier (as stated inthe just-described patent application).

A modulated component of the electrical signal (as measured by a lock-inamplifier) provides a measure of a property (such as thickness) oftraces 11A-11N. The modulated component of the electrical signal,obtained from measuring the change in reflectance of traces 11A-11N, issufficiently small relative to the overall electrical signal (due toreflectance of nonpolarized beam 12 by region 11) so that the overallelectrical signal can be used (as noted above) as a measure of aproperty (e.g. thickness) of layer 13. Therefore, a measure of themodulated component in act 123, and of the overall electrical signal (orits steady component) in act 122 together identify the presence of voidsin a structure, through comparison in act 124, and such measurements canbe performed in a single operation.

Instead of the just-described nonpolarized probe beam 12, a circularlyor elliptically polarized probe beam can also be used as describedherein for a nonpolarized beam (except that separate calibration isrequired for an elliptically polarized beam; specifically, in the caseof elliptically polarized light, the intensities in the two directionsare different: for example, if the ratio of intensity in the twodirections is 2:1 (parallel:perpendicular), then the parallel signalwill be twice as strong for the same reflectivity, and reflection in theparallel direction must be divided by 2 to compare to the reflection inthe perpendicular direction).

In one embodiment, a reflected portion of a nonpolarized probe beam 12is passed through a polarizer or a polarizing beam splitter to generateone or both components that have orthogonal polarization directions. Forexample, a polarizer may be used to select an individual polarizationdirection oriented perpendicular to traces 11A-11N as illustrated inFIG. 6, for measurement of a property of layer 13 in act 122. Thepolarizer can be a polarizing beam splitter available from Melles Griotof Irvine Calif. (see, for example part number 033 PBB 012).

When probe beam 12 (FIG. 6) is polarized perpendicular (i.e., 0=90°) totraces 11A-11N, then traces 11A-11N appear transparent to beam 12 due tothe orientation, as long as the wavelength exceeds the pitch. So,probe-beam 12 has energy that passes through layer 13 (that is at leastpartially transmissive), and the remaining energy of probe beam 12 isreflected (e.g., by surface 14 s; see FIG. 4) or absorbed. Thetransmitted portion passes between traces 11A-11N in the direction ofincidence and also in the opposite direction (after reflection), becausetraces 11A-11N act as a polarizer, as described in, e.g., the OpticsHandbook, pages 10-72 to 10-77.

For a given wavelength λ of probe beam 12, as the pitch p (FIG. 5) isreduced below λ (and the number of traces in region 11 is increasedcorrespondingly) a parallel polarized beam is reflected more effectivelyand a perpendicular polarized beam is transmitted more effectively.Specifically, an extinction ratio increases with reduction of pitch. So,in one embodiment, wavelength greater than pitch is used to yield alarge extinction ratio (e.g., greater than 2). When pitch is greaterthan or equal to 0.85 μm, the probe beam diameter Dp becomes on theorder of the width of the traces, so that eventually there is notransmission of the incident light, and instead there is fullreflection. For this reason, a larger probe beam diameter Dp may be usedin such cases, so that a number of traces are illuminated.

Thereafter, the polarizer may be rotated or replaced with anotherpolarizer to select a polarization direction parallel to traces 11A-11Nas illustrated by beam 12 in FIG. 7, for measurement of a property oftraces 11A-11N in act 123. When polarized parallel to traces 11A-11N, amajority of the energy of beam 12 is reflected by traces 11A-11N, andthe reflected portion is measured to determine a property of traces11A-11N, e.g., reflectance.

The predetermined frequency of modulation of heating beam 19 is selectedto be sufficiently small to cause a majority of the heat to transfer bydiffusion from region 11. Under such diffusive heat transfer conditions,the temperature of traces 11A-11N is approximately equal to (e.g.,within 90% of) the temperature of these same traces 11A-11N when heatedby an unmodulated beam (i.e., a beam having constant power, equal to theinstantaneous power of the modulated beam). For example, the modulationcan be sinusoidal between 0 and 50 milliwatts, i.e., P=50 sin (2π ft),where f is the modulation frequency. In such an example, at the timewhen the modulated power has an instantaneous value of 25 mW, thetemperature under heating beam 19 approximately equals (e.g., is no lessthan 90% of) the temperature obtained with a heating beam havingconstant power, e.g., 25 mW. In one example, heating beam 19 has awavelength of 0.83 microns, has an average power of 10 milliwatts, adiameter of 2 microns and is modulated at 2000 Hertz.

In one embodiment, the modulation frequency is selected to cause alltraces 11A-11N illuminated by heating beam 19 to be at substantially thesame temperature relative to one another (e.g., varying less than 10%between adjacent traces). Such a linear response condition occurs whenthe thermal wavelength λ (which is the wavelength of a thermal wave thatis formed in the structure) is at least an order of magnitude largerthan the diameter Dp of the illuminated region 11.

Therefore, when a heating beam 19 is modulated, the temperature T (andtherefore the reflectance) of traces 11A-11M is also modulated in phasewith modulation of the heating beam (under linear response conditions).Probe beam 12's reflected portion (which is sensed to generate anelectrical signal) is also modulated, in phase with modulation ofheating beam 19. The modulated electrical signal is detected by use of alock-in amplifier as stated in the patent application Ser. No.09/095,805. The modulated electrical signal can be used to identifyvariations in one or more materials of the traces (e.g., resistance perunit length which is indicative of cross-sectional area).

In an alternative embodiment, probe beam 12 is unpolarized, and apolarizing beam splitter separates unpolarized light reflected by traces11A-11N into two orthogonal polarization components, for instance,aligned parallel and perpendicular to traces 11A-11N. The parallel andperpendicular polarized components are then intercepted by separatephotodetectors to simultaneously measure their individual intensities.Moreover, a polarized probe beam 12 can be used in several ways,including, e.g., orienting beam 12 (FIG. 8) so that the electric fieldvector v thereof is at a predetermined angle θ (such as 0°, 90° or 45°)relative to the longitudinal direction 1B (FIG. 4) of traces 11A-11N.

In one embodiment, a damascene structure is formed in a wafer by aprocessing unit 201 that includes measuring apparatus 125 (FIG. 9) toevaluate the wafer for voids. Specifically, a patterning apparatus 120deposits and exposes a photoresist layer and an etching apparatus 121exposes and develops the photoresist layer 101 (FIG. 1) to form groovesin dielectric layer 102 on wafer 203 (FIG. 9). Thereafter, an etchingapparatus 121 etches through the patterned photoresist layer to formgrooves in an underlying insulation layer thereby to form wafer 204(FIG. 9). Next, a liner deposition apparatus 122 forms a barrier layerin the etched grooves and a conductive material 106 (FIG. 1) is blanketdeposited on wafer 205, by a deposition apparatus 123 thereby to formsubstrate 206. Next a polishing apparatus 124 is used to polish backlayer 106.

Thereafter, a measuring apparatus 125 in processing unit 201 is usedwith a programmed computer 126 to perform a process 300 illustrated inFIG. 10. Process 300 may also be performed at other times, depending onthe embodiment, e.g. as illustrated by arrows 210-213. At the conclusionof process 300, programmed computer 126 supplies a process parameter(used in the fabrication process) on a bus 215 that is coupled to eachof apparatuses 120-124 described above. A change in the processparameter can be determined automatically by software in programmedcomputer 126 (e.g. by performing a table look up), or can be entered bya human operator. Note that in one embodiment a single measurementoperation on wafer 207 detects the presence of voids, so that multiplemeasurement operations are not required. Note that at any point duringwafer fabrication of the type described above, a wafer can be subjectedto the measurement process 300.

In act 310 of process 300 (FIG. 10), a wafer is inserted into a waferaligner (in measuring apparatus 125 of FIG. 9), and traces formedtherein are oriented in a predetermined direction relative to a stage.Next, in act 301, a probe beam 12 that is either unpolarized orpolarized at a known orientation relative to the predetermined direction(i.e., relative to the traces) is generated, and illuminates the traces.Next, in act 306, a heating beam 19 that is modulated at theabove-described predetermined frequency is generated, and alsoilluminates the traces. Heating beam 19 may or may not be polarized (andif polarized, the orientation is parallel to the traces). Thereafter, inact 302, resistance per unit length (or thickness) of the array oftraces is measured, by sensing electromagnetic radiation that isreflected by the traces, that is polarized parallel to the traces, andthat is modulated at the predetermined frequency. A programmed computer126 (FIG. 9) checks if the measured property is within a predeterminedrange, and if not a process parameter is adjusted (e.g., via bus 115illustrated in FIG. 9) as illustrated by act 302 a.

Then, in act 303, a property of the layer 13 in which the array oftraces is embedded is measured, using a beam polarized perpendicular tothe traces. Computer 126 checks (in act 303) if the measured property iswithin a predetermined range, and if not another process parameter (oreven the same process parameter described above) is adjusted, asillustrated by act 303 a. Next, computer 126 compares (in act 304) thetwo measurements to one another, and if there is a large deviationindicating the presence of voids, yet another process parameter (or eventhe same parameter) is adjusted, as illustrated by act 304 a. Then theabove-described acts are repeated (in act 305) at a number of sites.

Although in the above paragraph a comparison between two differentmeasurements has been described in reference to act 304, the samemeasurement may be compared (i.e. to itself) to identify a defect.Specifically, a void may be detected by scanning a region to find asignal in excess of a base line, e.g. wherein a peak in the measuredsignal represents a void, and such peaks may be identified as localmaxima in a measured signal (regardless of the properties of theunderlying oxide and the related signal from the underlying oxide).Therefore instead of comparing two signals from the same location, thesame signal from different locations is compared in such an embodiment.

Note that the measurements can also be displayed to an operator (bycomputer 126) in a two-dimensional map of the wafer as illustrated inFIG. 11. For example, dies 5, 9, 11 and 12 may be shown highlighted(e.g., brightened, darkened or different color or hatched) to indicatethe presence of voids (if computer 126 is programmed to detect voids).There may be different types of highlighting (e.g., die 5 v/s dies 9,11, 12) to show the degree of variation in the presence of voids. Also,instead of highlighting, other mechanisms can be used, to convey thesame information, e.g. dies 9, 11 and 12 may be shown connected by acontour line (or shown shaded in a common color), to indicate thepresence of a certain percentage of voids, while die 5 may be connectedto other dice that contain the same percentage of voids, by anothercontour line (or color). Instead of using computer 126 to determine thepresence of voids, the measurement separations D in all dies may bedisplayed (in correspondingly varying shades of gray or color).

Such two-dimensional maps indicate variations across the productionwafer (e.g., dies 5, 9, 11 and 12) are located at the periphery of thewafer and therefore indicate a problem at the periphery. An example ofsuch a problem could occur due to voids forming in the metal traces,typically if dies all around the periphery fall outside of a controllimit (in the example dies 13-26 may fall with the control limit and soa different problem may be present). If several dies of a particularregion (e.g., dies 5, 9, 11 and 12 in the bottom right corner) areaffected, there may be a problem in that region, such as a number ofvoids in one or more of the traces in the illuminated region.

Instead of or in addition to the two-dimensional map of the waferillustrated in FIG. 11, the two measurements in each die can bedisplayed to an operator in the form of graphs as illustrated in FIGS.12A and 12B. Such graphs may be displayed instantaneously, as themeasurements are being made. Alternatively the graphs may be displayedin response to a query from an operator, e.g. after the two-dimensionalmap is displayed. FIG. 12A illustrates a scan across 0.14 μm wide linesthat are formed in two arrays: a first array with pitch (line-to-linespacing) of 0.3 μm illustrated in the left half of FIG. 12A and a secondarray with pitch of 0.28 μm illustrated in the right half of FIG. 12A.The large signals L1 and L2 come from a region between the two arrays.Note that the dielectric thickness signal S1 changes in the same manneras the metal resistance signal S2. Note that there are no voids in botharrays illustrated in FIG. 12A.

The effect of voiding at a different site in the same wafer isillustrated in the right half of FIG. 12B (and there is no voiding inthe left half). Note that in the right half of FIG. 12B, the metalresistance signal S2 spikes to very high values Sa-Sd consistent withhigh resistance, while the dielectric signal S1 shows far lessvariation. In such a case, the voiding is not within control limits, anda process parameter is adjusted, as illustrated by act 305 a. If aproduction wafer has no voids, one or more additional layers are formedon the wafer by the various apparatuses 120-124 (FIG. 9), and then thewafer is returned to the aligner (in act 310), and the measurement andcontrol acts 301-305 are repeated. Note that while forming theadditional layers, acts 301-305 and 310 can be performed on a differentproduction wafer.

FIG. 13A illustrates a wafer that was evaluated in the manner describedherein. Specifically 0.12 μm trenches were patterned in 8 kA thermaloxide on silicon wafers. A 250 Angstroms PVD Ta barrier was deposited.Trench voids were intentionally created by sputtering very thin (250Ang) copper seed in the trenches. Due to discontinuous Cu seed coverageat the bottom of some of the trenches, copper could not be electroplatedat those locations, resulting in sidewall voids. After CMP and anneal,wafers were evaluated to determine if method 300 (FIG. 10) could detectcopper voids.

Method 300 was used to scan two adjacent trench arrays, (labeled 300 nmand 280 nm) each of which had 0.14 μm target trench widths. The pitch oftrenches and lines in the arrays measured is 280 nm and 300 nm as shownin FIG. 13A. Each array is about 20 μm wide, as shown by width W in FIG.13A. The relative beam size of some tools is also shown in FIG. 13A toidentify the spatial relationship between the beam size, the arraywidth, and trench width. Apparatus 125 (FIG. 9) is capable of scanningacross an array because of the relatively small diameter of the heatingand probe beams. The beam spot may overlap from 5-10 trenches, dependingon the geometry of the wafer. As noted elsewhere, beams of spots thatoverlap only one trench or that overlap more than 10 trenches may alsobe used, depending on the embodiment.

Relatively speaking, small perturbations in the trenches are more easilydetected with small beams, as compared to large beams. After evaluationby apparatus 125, a reference wafer is normally evaluated by aconventional apparatus such as a scanning electron microscope (SEM). SEMphotos 1701-1704 shown in FIG. 13B are representative of voidsdiscovered in the measurement areas.

Results from four different samples measured by apparatus 125 are shownin a graph illustrated in FIG. 13B. Each of the four lines 1701-1704represents a different trench array. Each of lines 1701-1704 is a curvethat fits 1 μm interval individual measurements across one 20 μm wide,280 nm pitch array. The x-axis shows the measurement position, inmicrons. The y-axis represents the relative thermal/electricalresistance of the copper. A lower magnitude signal implies a largercross-sectional copper area. The curve 1704 labeled “No voids, standardtrenches” shows uniform and relatively low resistance across the trencharray, implying that there are no voids or other defects that decreasethe effective cross-sectional area of copper.

Trench voids and patterning defects result in higher base lines for thecurves 1701-1703 which are for the corresponding structures in thecross-sections 1711 (Large Voids), 1712 (Medium Voids), and 1713 (NoVoids Litho). The increased signal magnitude for these curves 1701-1703suggests a smaller cross-sectional copper area, thus higher resistance.These curves 1701-1703 also indicate non-uniformity across the array, orvariation in cross-sectional copper area from trench to trench.Cross-section FIB-SEMs 1711-1713 reveal voids and/or patterning defectsin the areas measured on these samples, both of which account for higherand non-uniform line resistance.

In one embodiment, a number of line scans are performed along thecorresponding number of lines that are parallel to one another, therebyto obtain an area map of the Damascene structure under evaluation. Inthe example illustrated in FIGS. 13C and 13D, area maps are of a voidedarray of 280 nm lines which shows large peaks on the edge on the leftside (e.g. between 6 μm-11 μm on the horizontal scale) of these graphs(both FIGS. 13C and 13D show the same information) due to the presentsof large voids. Note that an erosion pattern across the array is alsoevident in FIG. 13C. In FIG. 13C, the different shades of gray indicatedifferent amounts of voiding. The amounts of voiding for an area map mayalternatively be shown in a three dimensional graph as illustrated inFIG. 3D. Note that instead of gray scales, contour maps, or color graphscan also be used, depending on the implementation.

Acts 301-306 of method 300 can be performed by use of a void measurementapparatus 125 (FIG. 14) having two lasers 331 and 335 that create twobeams 301 and 302. Specifically, apparatus 125 includes a laser 331 forcreating a beam 301 of electromagnetic radiation at a predeterminedwavelength, such as infrared light, or ultraviolet light. Alternately, asource of X-rays, gamma rays, or radiation in the microwave or radiofrequencies can be used. In one embodiment, laser 331 is a AlGaAs diodelaser that emits electromagnetic radiation of wavelength 830 nm.Moreover the electromagnetic radiation emitted by laser 331 is linearlypolarized in this particular embodiment.

The electromagnetic radiation created by laser 331 is transmittedthrough an optical fiber 332 to a collimator 333 that emits heating beam301. In one implementation, heating beam 301 has a maximum power of, forexample, 100 milliwatts. Apparatus 125 also includes lenses 304A and304B that adjust the size of beam 101 to fill the aperture of anobjective lens 315 also included in apparatus 125.

Apparatus 125 further includes a second laser 335 that creates a beam302 of electromagnetic radiation used to measure a change in reflectanceof traces 11A-11N in response to change in power of heating beam 301. Inone implementation, laser 335 is an InGaAs diode laser that emitselectromagnetic radiation of wavelength 980 nm. The electromagneticradiation created by laser 335 is transferred by an optical fiber 336 toanother collimator 307 also included in apparatus 125. Collimator 307emits probe beam 302 having a maximum power of, for example, 7milliwatts. Therefore, probe beam 302 has a power that is an order ofmagnitude smaller than the power of heating beam 301, so that conductivetraces 11A-11N are not noticeably heated by probe beam 302. Moreover,collimator 307 emits electromagnetic radiation that is circularlypolarized, so that beam 302 has components polarized in the parallel andperpendicular directions relative to traces 11A-11N.

Apparatus 125 also includes lenses 308A and 308B that adjust the size ofprobe beam 302 to fill the aperture of objective lens 315 (describedabove). Apparatus 125 also includes a dichroic beam splitter 310 thatcombines heating beam 301 and probe beam 302 to form a combined beam311. Combined beam 311 passes through beam splitters 312 and 314 thatare also included in apparatus 125, to an objective lens 315. Objectivelens 315 can be, for example, a 0.9 NA, 100× objective lens availablefrom Nikon of Yokohama, Japan.

A portion of combined beam 311 is deflected to a photodetector 313, suchas part number J16-8SP-RO5m-HS from EG&G Judson of Montgomeryville, Pa.,USA. Photodetector 313 is used to verify the alignment of combined beam311 with respect to wafer 305, and to measure the incident power of oneor both of beams 301 and 302. Apparatus 125 also includes a beamsplitter 314 that diverts 10% of combined beam 311 to a focusing lens317 and a camera 318. Camera 318 is used to observe beams 301 and 302(FIG. 1B) on wafer 305, in order to focus combined beam 311 (FIG. 3)within region 111R (FIG. 1B) on wafer 305.

Light reflected from wafer 305 passes back through objective lens 315and through beam splitter 312. Beam splitter 312 sends 50% of thereflected light through a filter 319. Filter 319 is a narrow band filterthat removes the reflected portion of heating beam 303 while passing thereflected portion of probe beam 309. Thereafter, a polarizing beamsplitter 338 passes one polarization component to detector 340 anddeflects the other polarization component to detector 339. Detectors 339and 340 simultaneously provide measurements, of the metal property andthe dielectric property.

A signal from detector 340 (of the metal property) is amplified by atransimpedance amplifier 324 and a voltage amplifier 323 that providesthe amplified signal to a lock-in amplifier 322. Lock-in amplifier 322includes an oscillator as a frequency source that is used to detect thepower of the reflected portion of probe beam 302 modulated at thepredetermined frequency. The frequency source in lock-in amplifier 322also provides a frequency signal on a line 321M to a laser driver 321.Laser driver 321 uses the frequency signal on line 321M to drive laser331 at the predetermined frequency that is sufficiently low to modulatethe amplitude of heating beam 301 to ensure heat transfer by diffusionas described herein.

In one embodiment, filter 319 is mounted on an actuator 337 that can beoperated to remove filter 319 from the path of the reflected portion ofthe heating beam towards the polarizing beam splitter 338. When soremoved, the reflectance of the heating beam is also measured, to obtaina second measure of the thickness of the dielectric layer in wafer 305.The second measure is needed because the reflectance of the dielectriclayer is periodic in the ratio of thickness to wavelength. Therefore, atcertain thicknesses, the reflection signal at the wavelength (e.g. 980nm) of the probe beam is at a maximum or minimum of the cosine, and thesensitivity to changes in thickness is small. In this case, themeasurement of dielectric thickness is taken using reflection of theheating beam.

Alternatively, a variable wavelength light source, such as a white lightsource, and a monochrometer may be added into the path of the reflectedelectromagnetic radiation, and the reflectivity measured at multiplewavelengths. Such measurement removes any ambiguity that may occurthrough use of a single wavelength probe beam to measure dielectricthickness.

Note that instead of laser 335 generating a circularly polarized beam302, another laser that generates a linearly polarized beam can be usedwith a half wave plate that is mounted on an actuator to change thepolarization direction. In such an embodiment, polarizing beam splitter338 is not used, and instead the measurements of metal property anddielectric property are made sequentially.

Numerous modifications and adaptations of the above-describedembodiments will become apparent to a person skilled in the art of usinglasers to measure semiconductor properties. For example, in analternative embodiment, instead of using a laser to generate heatingbeam 301 to change peak temperature Tp of traces 11A-11N, another heatsource (such as an electron gun) is used to modulate the temperature.Use of electrons in beam 301 instead of photons allows the diameter ofbeam 301 to be made smaller than possible when using photons. However,use of electrons in beam 301 requires measurement apparatus to include avacuum chamber to contain the electron source.

As noted above, the just-described regions can be inside a singlestructure or spread across multiple such structures (e.g., in areference structure that has known material properties, and a productionstructure that is currently being fabricated and whose properties areyet to be determined, or even multiple production structures). Note thatacts 121, 124 and 125 described above in reference to FIG. 2 areoptional, and may or may not be performed, depending on the embodiment.For example, the generated electrical signals may be manually evaluated.Alternatively, such evaluations (manual or automatic) may be performedindependent of the fabrication processes of the structures.

Moreover, the above-described method and apparatus can be used withtraces 11A-11N of any metal (such as copper or aluminum) or any silicide(such as titanium, cobalt, or platinum), irrespective of whether or notthe traces have been annealed. Furthermore, depending on the embodiment,the thickness of layer 13 is measured in one implementation of act 122by illuminating traces 11A-11N with polarized white light, and measuringthe color of the reflected light, e.g. with a camera (and optionally animage processor). Films (such as layer 13 in FIG. 4) that aresufficiently thin (e.g. about 1-2 μm thick) have a reflectance that is afunction of wavelength, and therefore reflect light of a color thatdepends on the thickness (e.g. thickness T+H in FIG. 1).

The following table indicates the change in color of the reflected lightas a function of thickness of the underlying layer (this table isprovided as an example. It is based on a single layer silicon dioxidecoating on an unpatterned wafer. Similar tables can be generated forother material structures by, for example, making structures ofdiffering thickness). Film Thickness (μm) Color of reflected light 0.05Tan 0.07 Brown 0.10 Dark violet to red violet 0.12 Royal blue 0.15 Lightblue to metallic blue 0.17 Metallic to very light yellow green 0.20Light gold or yellow slightly metallic 0.22 Gold with slight yelloworange 0.25 Orange to melon 0.27 Red violet 0.30 Blue to violet blue0.31 Blue 0.32 Blue to blue green 0.34 Light green 0.35 Green to yellowgreen 0.36 Yellow green 0.37 Green yellow 0.39 Yellow 0.41 Light orange0.42 Carnation pink 0.44 Violet red 0.46 Red violet 0.47 Violet 0.48Blue violet 0.49 Blue 0.50 Blue green 0.52 Green (broad) 0.54 Yellowgreen 0.56 Green yellow 0.57 Yellow to “yellowish” (not yellow but is inthe position where yellow is to be expected. At times it appears to belight creamy gray or metallic) 0.58 Light orange or yellow to pinkborderline 0.60 Carnation pink 0.63 Violet red 0.68 “Bluish” (Not bluebut borderline between violet and blue green. It appears more like amixture between violet red and blue green and looks grayish) 0.72 Bluegreen to green (quite broad) 0.77 “yellowish” 0.80 Orange (rather broadfor orange 0.82 Salmon 0.85 Dull, light red violet 0.86 Violet 0.87 Blueviolet 0.89 Blue 0.92 Blue green 0.95 Dull yellow green 0.97 Yellow to“yellowish” 0.99 Orange 1.00 Carnation pink 1.02 Violet red 1.05 Redviolet 1.06 Violet 1.07 Blue violet 1.10 Green 1.11 Yellow green 1.12Green 1.18 Violet 1.19 Red violet 1.21 Violet red 1.24 Carnation pink tosalmon 1.25 Orange 1.28 “Yellowish” 1.33 Sky blue to green blue 1.40Orange 1.45 Violet 1.46 Blue violet 1.50 Blue 1.54 Dull yellow green

For such a color measurement, the white light is polarized, as describedin U.S. patent application, Ser. No. 09/521,232 [attorney docket numberM-7850 US] incorporated by reference above. Referring to FIG. 14, thefollowing changes are made to implement this method: a beam splittercube is inserted in the beam between lens 317 and beam splitter 314, anda white light source (such as a halogen lamp) is added to illuminate thenew beam splitter. The new beam splitter injects the white light intothe beam path.

Once the color is measured (either by human observation or by an opticalinstrument), the above table or a similar table may be used with themeasured color to look up the thickness t. Note that thickness lookup isnot necessary to measure voids, and in one embodiment, the above tableis not used in the measurement process at all. Instead, such anembodiment monitors only uniformity of the oxide's thickness. However,thickness look-up could be performed in other embodiments, since itprovides a rapid way to determine dielectric thickness over large areas,with high resolution (resolution is determined by the number of pixelsin the image). Also, the above color table is only valid for a singlelayer of silicon dioxide over silicon. In practice, other dielectricmaterials might be used, or multiple layer films would be present, andthe above table would have to be modified. An alternate embodiment readsout the color (e.g. red, green and blue values) from the camera, andcorrelates the read color to the thickness, which has been previouslymeasured using a conventional measurement system such as anellipsometer.

Another embodiment performs the following acts:

-   -   1. Thickness and index of refraction of each layer in the film        is measured at a point outside but near the metal array, using a        conventional instrument such as an ellipsometer.    -   2. The computer contains a model that may be created as follows.        Once the thickness and index of refraction of each layer are        known, the reflection at a known wavelength is calculated using        conventional formulae. See, for example, Handbook of Optics,        W.G. Driscoll and W. Vaughan editors, McGraw-Hill (New York)        1978, pp. 8-42 and 8-43 . The color may be predicted using        colormetric methods described in chapter 9 of the same book that        is incorporated by reference herein in its entirety. The model        which predicts the red, green and blue content of the reflection        as a function of the measured index of refraction of each layer,        the thickness of each underlying layer, and the thickness of the        top layer. The measured color of the array is then compared to        the predicted to determine the thickness of the top layer. This        is the only layer that would vary in a polishing process, as the        polish would not affect underlying layers.

Therefore, in certain embodiments, a relative difference in thickness ismeasured (either qualitatively or quantitatively) by comparing thecolors obtained from two (or more) different regions of a structure,thereby to obtain a corresponding change in thickness of the layer inwhich the traces are embedded (only this layer would change with theprocess; the underlying layers are constant).

In one implementation, one or more measurements of the type describedherein are made by a circuit 600 (FIG. 15) that uses a photodiode (e.g.either of diodes D1 and D2 to generate a current (e.g. 1-2 milliamps) inresponse to the intensity of light incident on the photodiode.Thereafter, an amplifier U4 (FIG. 15) converts the current from thephotodiode into a voltage (e.g. 2-4 volts). Amplifier U4 is coupled to afilter U10 that filters out high frequency noise (e.g. from power lines;e.g. U10 may suppress any signal outside the frequency range 100 Hz to 5KHz).

Thereafter, an amplifier U11 amplifies the varying component (alsocalled “ac” component) of a measured signal by a gain that is selectableby the user (e.g. the gain may be any one of 1, 2, 4, 8, 16, 32, 64 and128). The gain may be selected by the user depending on the structurethat is currently under examination, and the type of signals beingobtained from the measurement. If necessary, an optional 10× gainamplifier may be used to further amplify the measured signal. Theresulting signal is provided to a lock-in amplifier for processing asdescribed herein.

In another implementation, a signal from another photodiode D2 isamplified (as described above, but by amplifier U7). In addition tosumming the measured signals, these signals can be compared to oneanother, e.g. by an amplifier U1 which provides a difference signal. Thedifference signal is proportional to a property of the wafer, such assurface roughness.

In an alternative implementation, signals from each of amplifiers U4 andU7 are supplied to a summer (not shown) that in turn provides to filterU10 a signal that is the sum of the two signals obtained from the twophotodiodes D1 and D2, for use as described herein.

Depending on the embodiment, beams 12 and 19 need not be coincident andin fact may be separated from one another e.g. in the longitudinaldirection of traces 11A-11N if the effect of heating beam 19 can bemeasured across the separation distance. Moreover, the two signals S1and S2 need not be measured simultaneously at the same location, andinstead each of these signals can be measured individually and compared(or otherwise processed) to detect voids at a different time. Forexample, signal S2 may be measured several milliseconds after signal S1and yet presence of voids detected as long as values for the samelocation are compared.

In another embodiment, acts of the type described herein are applied toa single metal line, either as an isolated line or as a line in an arrayof pitch comparable to or larger than the spot size. In this embodiment,measurements are performed using a scanning method, so that the positionof the beam relative to the line is known. The scanning methodeliminates the need for a vision system to perform automatic alignment.So a vision system that is unable to perform an alignment to therequired tolerance can be used in this embodiment.

Specifically, probe beam 12 and heating beam 19 are initially coincidentat a first position 902 a (FIG. 16A) that is located sufficiently faraway from trace 901 so that beams 12 and 19 do not overlap trace 901. Atposition 902 a, a first signal 903 (see FIG. 16B) for the metalresistance measurement is zero, and a second signal 906 for thereflection measurement relating to the dielectric thickness has anon-zero value. Next, the two beams are scanned along arrow 907 (FIG.16A), while remaining coincident with one another. First signal 903reaches a maximum (FIG. 16B) when beams 12 and 19 overlay trace 901(FIG. 16A) at a position 902 b that is in the middle of trace 901, andfalls back to zero (FIG. 16B) as beams 12 and 19 reach position 902 c(FIG. 16A). Similarly, because trace 901 absorbs light, second signal906 drops to a minimum at position 902 b and returns to the originalvalue at the other side of trace 901, at position 902 c. In this case,signal profile 903 is read and compared to reflection profile 906.Changes in the relative height of signal 903 correspond to a potentialvoiding problem—that is, a void creates an obstruction in heat flow thatincreases the level of signal 903. If there was a void, then signal 903in FIG. 16B has a higher value than a similar scan across anotherportion of the line lacking a void.

In the just-described method, positions 902 a-902 c were collinear (i.e.along a straight line). In a variant of the just-described method, beams12 and 19 are scanned from a position 902 a near trace 901 to a position902 b over trace 901, and thereafter along the length of trace 901 to aposition 902 d. Therefore, positions 902 a, 902 b and 902 d are notcollinear. In such an embodiment, first signal 904 (FIG. 16D) climbs ina manner similar to first signal 903 (FIG. 16B) until position 902 b. Atposition 902 b, the scan direction is the changed and beams 12 and 19are moved along the length of trace 901, to position 902 d. If there areno voids, then first signal 904 and second signal 905 remain constantduring the travel along trace 901. However, any peaks 906 a and 906 b infirst signal 904 during travel along trace 901 indicate a void. Ifsecond signal 905 trends up, as illustrated by dashed line 909, then thebeams may be drifting off from trace 901. Therefore, the second signal905 can be used to ensure that the beams remain aligned to trace 901.

Therefore, as illustrated in FIGS. 16A-16D, certain embodiments measuresignals from a trace which is separated from other traces by at leastthe diameter of the beams, and interpret changes in the reflectionproperties as indicating the presence of voids under the trace. In theseembodiments, the measurements are not in any sense averaged overmultiple traces, and instead are actual measurements for the individualtrace.

Moreover, voids in a via chain may also be detected as discussed in therelated U.S. patent application, Ser. No. 10/090,287 entitled“IDENTIFYING DEFECTS IN A CONDUCTIVE STRUCTURE OF A WAFER, BASED ON HEATTRANSFER THERETHROUGH” filed Mar. 1, 2002, by Peter G. Borden andJi-Ping Li. One or more acts of the type described therein may be usedwith one or more acts described herein, depending on the embodiment.Moreover, any acts of the type described herein may be performed in anyorder relative to one another, although depending on the embodiment,some acts may be performed in a specified order, for use in certainapplications.

Furthermore, voids can be detected by measuring the dame property atdifferent locations, followed by comparison of the measurements from thedifferent locations. For example, if signal S1 is being measured 9seeFIG. 3), then simply a change dS1 can be used to identify a deviationfrom baseline.

Numerous modifications and adaptations of the above-describedembodiments, implementations, and examples are encompassed by theattached claims.

1. A method for evaluating a structure that has a plurality of layers,the method comprising: measuring a first property of a first layer inthe structure, to obtain a first measurement; measuring a secondproperty of a second layer in the structure, to obtain a secondmeasurement; and using the first measurement and the second measurementto determine existence of a void.
 2. The method of claim 1 wherein: saidfirst layer comprises a plurality of traces of conductive material andsaid first property is one of (thickness and cross-section); and saidsecond layer comprises a dielectric material and said second property isthickness.
 3. The method of claim 1 wherein said using comprises:displaying the first measurement and the second measurement in a graphas a function of location.
 4. The method of claim 1 wherein said usingcomprises: comparing the first measurement with the second measurement;and identifying a current location as having the void depending on anoutcome of said comparing.
 5. The method of claim 4 wherein saidcomparing comprises: checking if the separation of said first and secondmeasurements is greater than a predetermined amount.
 6. The method ofclaim 4 wherein said comparing comprises: comparing separation betweenthe first measurement and the second measurement at the current locationwith a corresponding separation in another location.
 7. The method ofclaim 4 wherein said comparing comprises: checking if separation betweenthe first measurement and the second measurement at the current locationis approximately an order of magnitude greater than a correspondingseparation in another location.
 8. The method of claim 7 wherein: themeasurings at the locations are performed during a scan in a directiontransverse to the plurality of traces and the group of traces isdifferent in each of the locations.
 9. The method of claim 1 wherein:the first property is related to a dimension of the first layer; and thesecond property is related to a dimension of the second layer.
 10. Themethod of claim 1 wherein: the first layer comprises a plurality oftraces of conductive material supported by the second layer; and themeasuring of first property is performed across a group of traces,thereby to obtain in the first measurement an average measure of thefirst property across traces in the group.
 11. The method of claim 10wherein the measuring of first property comprises: illuminating thestructure with a beam comprising electromagnetic radiation having awavelength greater than a pitch between two adjacent traces in saidgroup.
 12. The method of claim 11 wherein: the electromagnetic radiationis polarized parallel to the plurality of traces.
 13. The method ofclaim 11 wherein the beam is hereinafter “first beam,” and the measuringof first property further comprises: illuminating the structure with asecond beam having intensity modulated at a predetermined frequency, thepredetermined frequency being sufficiently small to ensure that heatgenerated in a region illuminated by the second beam transfers out ofthe region by diffusion.
 14. The method of claim 13 wherein: the secondbeam comprises electromagnetic radiation polarized in a directionparallel to the plurality of traces.
 15. The method of claim 14 whereinthe measuring of second property further comprises: illuminating thetraces with a beam comprising electromagnetic radiation polarizedperpendicular to the plurality of traces.
 16. The method of claim 11wherein the beam is hereinafter “first beam,” and the measuring of firstproperty further comprises: focusing a second beam of electrons, saidsecond beam having intensity modulated at a predetermined frequency. 17.The method of claim 11 wherein: the first property is resistance perunit length of said traces; and the measuring of first property includesmeasuring average reflectance of said traces.
 18. The method of claim 1wherein the measuring of second property comprises: measuringelectromagnetic radiation reflected by the second layer, theelectromagnetic radiation being polarized perpendicular to a pluralityof traces in the first layer.
 19. A method for evaluating a structurehaving at least a plurality of traces and a layer in contact with saidtraces, at least two traces in the plurality being each at leastsubstantially parallel to the other, the method comprising: illuminatingthe structure with a first beam comprising electromagnetic radiationpolarized parallel to the two traces; illuminating the structure with asecond beam having intensity modulated at a predetermined frequency, thepredetermined frequency being sufficiently small to ensure that heatgenerated in a region illuminated by the second beam transfers out ofthe region by diffusion; and measuring an attribute of saidelectromagnetic radiation reflected by at least said two traces andmodulated at the predetermined frequency.